1. Field of the Invention
The present invention relates generally to the production of shaped, tempered sheets of glass and, more particularly, to an improved method of and apparatus for shaping and heat treating relatively thin glass sheets to shapes having a longitudinal component of curvature about an axis extending transversely of the path traversed by the shaped glass sheet during its shaping.
Shaped glass sheets are widely used as side windows in vehicles, such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped windows to damage resulting from impact.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. To promote efficient and large scale production, discrete glass sheets are conventionally heated, bent and cooled while being moved continuously along a fixed path and successively through a heating section, a roll forming section, a quenching section and a cooling section. To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or interior thereof above a deformation temperature upon being exposed initially to the quenching medium at the quenching section. The residual heat remaining in glass sheets of previous commercial thicknesses, such as those having nominal thicknesses ranging from 4.5 millimeters to 6 millimeters, is generally sufficient after shaping for immediate advancement to the tempering area and exposure to the quenching medium. Thus, the heat initially imparted to a relatively thick glass sheet to bring it to proper temperature for shaping can also be utilized in the final heat treating operation.
However, within the last several years, considerable emphasis has been placed on the use of thinner and thinner glass sheets for automobile side windows as a means of reducing overall weight of the autos as a means to obtain better fuel mileage. This has posed problems in shaping and tempering, due to the lesser ability of the thinner sheets to retain heat and the aforementioned conventional process of bending and treating glass sheets does not lend itself to the processing of these relatively thin sheets, such as those having nominal thicknesses ranging from less than 3 millimeters to 4 millimeters (90 mils to 160 mils). As the thickness of the glass decreases, the rate of heat loss increases and the heat initially imparted to such thin sheets is quickly dissipated upon leaving the heating atmosphere of the furnace and during the relatively cool bending cycle. Attempts to solve these problems by initially overheating the thin glass sheets have not been successful because of the consequent loss of control of the glass shaping process and the degradation of the surface quality of the finished glass as a result of heat stains, roll ripple distortion, and the imposition of roll marks in the surface of the heat-softened glass sheet.
Consequently, roll forming has been developed as a technique for shaping and tempering glass sheets at a high production rate. One of the benefits of the roll forming process is the rapid removal of each individual glass sheet from the heating section or furnace through the shaping section and into the quenching section. In the roll forming method, glass sheets are conveyed without stopping through heating, shaping, and tempering sections along high speed glass sheet conveyor means to drastically reduce the time needed to traverse the distance between the exit of the heating section or furnace to the tempering or quenching section to a minimum, preferably under 5 seconds. Under such circumstances, thin glass sheets can be tempered by quenching without imparting such a high initial temperature at the furnace that shape control and control of surface quality is lost as a consequence of insuring that the temperature at the core of each glass sheet does not cool to below the minumum temperature needed on arrival at the quenching section to assure adequate temper.
Quenching or tempering medium is applied against the opposite major surfaces of the shaped glass sheets at the quenching section. Beyond the quenching section, tempered glass sheets are cooled to handling temperature as they traverse a cooling section where they are further cooled.
In order to be practical for commercial purposes, the roll forming section of roll forming apparatus includes a series of forming rolls whose vertical positions can be adjusted so as to provide a series of forming rolls disposed in longitudinally spaced relation along an arcuate longitudinal path of approximately constant radius of curvature, which radius of curvature can be adjusted to handle sheets of different longitudinal shapes. The distance between the end of the furnace conveyor section and the beginning of fixed, horizontally aligned conveyor rolls in the cooling section of the apparatus where the glass sheets have surfaces sufficiently hard to withstand surface damage on contact with misaligned rolls is fixed. Consequently, in making symmetrical longitudinal bends of a predetermined radius of curvature, the glass sheets have to traverse an arcuate path comprising a portion of the circumference of a circle before they reach the fixed, horizontally aligned conveyor rolls. This arcuate path comprises a downstream portion having an upward slope symmetrical with a downward slope at the upstream end of the arcuate path for said predetermined radius of curvature.
Typical roll forming apparatus for shaping glass sheets to a curved configuration comprising a longitudinal curvature component about an axis of bending that extends transversely of the path of glass sheet movement comprises a roll conveyor system comprising a furnace conveyor extending through a furnace along a horizontal path at one level, a cooling conveyor extending through a cooling section (from a conveyor section beginning at a location where the glass surfaces are hardened) along a horizontal path at said level and a forming conveyor and a quenching conveyor defining an arcuate longitudinal path of substantially constant radius of curvature of concave elevation interconnecting the furnace conveyor with the cooling conveyor. The upstream portion of the forming conveyor is sloped downward and the downstream portion of the quenching conveyor is sloped upward. Frequently, slippage occurs in the upward sloped portion of the quenching conveyor, particularly when shaping glass sheets to a longitudinal component of curvature that has a sufficiently short radius of curvature that the upward slope of the arc of the curve at the downstream portion thereof is so steep that the shaped glass cannot be propelled upward by friction of the rotating rolls in the downstream portion of the quenching conveyor.
The problems associated with shaping thin glass sheets to shapes incorporating a longitudinal curvature component about an axis transverse to the path of glass sheet movement will be understood better in the light of a description of the prior art that follows.
2. Description of the Prior Art
Many patents have been issued on roll forming.
Drake U.S. Pat. No. 2,348,887 moves heated glass sheets between a pair of aligned pressure rolls 32 and 33 of cylindrical configuration which force the bottom surfaces of the glass sheets to ride over a series of spaced bending rolls 31 of cylindrical configuration mounted for rotation along spaced lines that extend transversely of a longitudinally curved path corresponding to the shape desired for the bent glass sheets. The shapes imparted to the moving glass sheets are limited to cylindrical curvatures of uniform rather large radius about an axis transverse to the path of glass movement. Once the trailing end of a glass sheet passes beyond the pressure rolls, it becomes difficult to prevent slippage between the sheet and the rotating rolls.
Frank U.S. Pat. Nos. 3,701,644; 3,856,499; 3,871,855; 3,891,420; 3,929,441; 3,934,996; 3,992,181 and 4,043,783, and Knapp U.S. Pat. No. 3,896,269 disclose roll forming apparatus capable of shaping a succession of discrete moving glass sheets to either simple shapes provided with one component of shape about either an axis extending longitudinally of the path of glass sheet movement or about an axis extending transversely thereof or compound shapes involving various combinations of the first two components conforming to said simple shapes. The roll forming apparatus of this group of patents uses sets of forming rolls that move transverse to the path of glass sheets therebetween during shaping.